1. Field of the Invention
The present invention relates generally to methods for preparing particles (advantageously submicron-sized particles) of pyrithione salts, and, more specifically, to methods of preparing such particles using de-agglomeration procedures subsequent to production of the particles. The present invention also relates to products made with these particles.
2. Description of the Related Art
Polyvalent metal salts of pyrithione (also known as 1-hydroxy-2-pyridinethione; 2-pyridinethiol-1-oxide; 2-pyridinethione; 2-mercaptopyridine-N-oxide; pyridinethione; and pyridinethione-N-oxide) are known to be effective biocidal agents, and are widely used as fungicides and bactericides in paints and personal care products such as anti-dandruff shampoos. The polyvalent metal salts of pyrithione are only sparingly soluble in water and include magnesium pyrithione, barium pyrithione, bismuth pyrithione, strontium pyrithione, copper pyrithione, zinc pyrithione, cadmium pyrithione, and zirconium pyrithione. The most widely used divalent pyrithione salts are zinc pyrithione and copper pyrithione.
Zinc and copper pyrithione are useful as antimicrobial agents and are active against gram-positive and negative bacteria, fungi, and yeasts. Zinc pyrithione is used as an antidandruff component in shampoos, while technical suspensions of zinc pyrithione and/or copper pyrithione are used as preservatives in paints and polymers. Synthesis of polyvalent pyrithione salts are described in U.S. Pat. No. 2,809,971 to Berstein et al. Other patents disclosing similar compounds and processes for making them include U.S. Pat. Nos. 2,786,847; 3,589,999; 3,590,035; 3,773,770.
Known methods for producing insoluble polyvalent salts of pyrithione result in platelet-shaped (or other irregular shaped) particles having an average size greater than 1 micrometer (μm), and more frequently in the range of 3 to 5 μm. These particles are either used directly, or can be converted into smaller particles. Smaller particles of pyrithione salts (i.e., less than 1 micrometer or “submicron”) are often desired because they more easily form suspensions, emulsions, or dispersions, and provide a larger surface area for enhanced biocidal activity. In addition, smaller particles, particularly in the low submicron range (e.g., below about 0.2 μm are believed to be semi-transparent to light, and below 0.1 μm will be transparent to light). This transparency provides the opportunity to manufacture “clear” products, such as clear shampoos and soaps, that are popular in the marketplace today, while providing the larger surface area desired for enhanced biocidal efficacy.
Submicron-sized particles of pyrithione salts are usually generated by a separate mechanical manipulation step (e.g., grinding or crushing) of larger particles or crystals that are made by conventional processes. For example, European Patent Application No. 70046 describes a process for the preparation of zinc pyrithione using organic solvents. This process results in production of large crystals of zinc pyrithione. A separate, optional grinding step is used to grind the large crystals and produce zinc pyrithione particles of smaller size. In another example, U.S. Pat. No. 4,670,430 describes a process of making zinc pyrithione particles with a median size of about 0.2 μm or less by mechanical grinding of larger particles of zinc pyrithione to the desired submicron size. Unfortunately, mechanical grinding of large pyrithione particles into a submicron sized pyrithione particles tends to not produce submicron-sized particles having a desired uniform size, shape and narrow particle size distribution. Such desired parameters are important since they are useful in rendering the behavior of the particles in consumer products, such as shampoos and coatings, predictable. In addition, grinding generally results in substantial loss of useful product and is costly in terms of the equipment, time, and energy required to provide the ground particles. Moreover, a desired particle shape for pyrithione particles, such as rods, needles, or other shapes with potentially enhanced biocidal activity, cannot easily be selected and produced by using grinding methodology.
Submicron-sized particles of pyrithione salts made by the methods of the prior art also suffer from severe agglomeration in which many of the submicron-sized particles bond together through noncovalent interactions to form larger particles of greater than 1 micron in size. Due to high mass, these large agglomerated particles tend to settle out of most consumer products over time and result in a hard packed layer of pyrithione salt that is difficult to re-disperse.
What is needed in the art is a method for producing non-agglomerated or de-agglomerated particles, advantageously having a submicron size or larger, of pyrithione salts possessing a uniform size, shape and/or size distribution. Desirably, the particles, incorporated into a solution, suspension or dispersion, are stable against settling out or agglomerate over time during shipping or storage prior to use. In addition, it is desired that the particles do not exhibit the damage that is typically associated with mechanical grinding. The present invention is believed to provide answers to these needs.